This was a double-blinded, randomized, placebo-controlled evaluation of the prophylactic antimalarial activity of atovaquone in sixteen healthy volunteers with induced Plasmodium falciparum malaria. The study was conducted on an outpatient basis, and had three arms: a) six volunteers receiving 750 mg qd starting the day before challenge; b) six volunteers receiving a single dose of 250 mg the day before challenge; and c) four volunteers receiving placebo. Volunteers were challenged on October 1, 1996 with the NF54 strain of chloroquine-sensitive P. falciparum, by the bites of infected Anopheles stephensi mosqitoes. Efficacy was evaluated by serial blood examinations (on 35 occasions over a three month period) for malaria parasites and by monitoring for symptoms and signs of infection. Parasites were cultured and their sensitivity to atovaquone was determined. Four subjects developed patent malaria and were treated successfully with chloroquine. The code will be broken and data analyzed when the study is completed and all results have been reviewed.